Croquet is an open source software platform to develop and deliver deeply collaborative multiuser online programs. It has a network architecture supporting communication, collaboration, resource sharing, and synchronous computation among users. It provides a flexible framework in which most user interface concepts can be prototyped and deployed to create powerful, collaborative multiuser 2D and 3D programs and simulations. It can be used to make scalable collaborative data visualizations, virtual learning and problem solving environments, 3D wikis, online gaming environments (MMORPGs), and privately maintained and interconnected multiuser virtual environments.